Secondary Instability of Unstable Baroclinic Waves

In a previous quasigeostrophic study the modes of instability of a neutral baroclinic wave were found to depend, in structure, phase speed and growth, on the amplitude of the primary wave and to give rise to a form of absolute instability when the primary wave is deep enough. A primitive equation model is here used to verify the robustness of those results in a more realistic context. Because of the frontogenetic process intrisic in the amplification of a baroclinic wave the study must proceed from an unstable primary wave, and the secondary instability is modified by the growth of the primary wave. Modes with the same qualitative characteristics as the QG ones are found, but their growth is slower than the growth rate of the primary wave itself, and therefore they do not emerge in the free evolution of the primary wave. The possibility remains to be explored that a perturbation forced e.g. by an orographic obstacle projects on those slower modes, which would then determine the stucture and evolution of the orographic perturbation